1. Field of the Invention
The invention relates to an electrohydraulic device for generating a force to be applied to a vertically movable piston rod that is connected with the piston of a cylinder, the device including a first centrifugal pump disposed in the portion of the cylinder facing away from the piston rod and connected with an electric motor by way of a shaft, a housing for the cylinder and the centrifugal pump, with the housing defining a reservoir for the operating fluid and being provided with passages for the shaft and the piston rod. In dependence on the direction of rotation of the electric motor, the operating fluid is pumped through a pressure conduit from a cylinder chamber above the piston into a portion of the cylinder chamber below the piston or in the opposite direction, thus causing the piston to be moved by the action of pressure and to generate forces at the piston rod.
2. Description of the Related Art
Such a hydraulic device is known from German Utility Model Patent DE-GM 1,690,019. This device has a horizontally arranged impeller wheel equipped with guide vanes and driven by an electric motor. In addition, a valve body is disposed above the impeller wheel and is pivoted by the tangential forces of the operating fluid into one of two fixed positions depending on the direction of rotation of the electric motor. In this way, the operating fluid is given its direction of flow. In the one position, the operating fluid is pumped from a region above the piston into a region below the piston and in the other position, when the motor rotates in the opposite direction, the operating fluid flows in the opposite direction. Depending on the direction of flow, the piston is moved either upward or downward. The drawback is that the manufacture of the valve body requires rather complicated and precision working so that it reliably determines the flow direction.
German Patent 842,440 also discloses an electrohydraulic adjustment device including a motor and a pump. To raise a piston equipped with a piston rod, operating fluid is pumped from a cylinder that is open toward the space above the piston into a pressure chamber below the piston. Thus the piston is hydraulically moved upward.
After the motor is switched off, tensioned counter-springs or raised counter-weights push the piston back into its lower starting position. A drawback of this prior art device is that it generates only an upwardly directed pressure force. If the motor is switched off and during the transition from the upper end position into the lower end position the device itself does not generate force, rather the downward movement of the piston is effected on the basis of a force that acts on the piston rod from the outside.